1. Field of the Invention
The present invention relates to systems used in satellite communication. More particularly, the present invention provides for the reduction of noise in satellite signals in carrier-in-carrier satellite communications.
2. Background of the Related Art
Each direction of a conventional duplex radio link typically uses different carrier frequencies. If the same frequency was used for both directions, the transmit signal, which can be 4-5 orders of magnitude larger, can swamp the received signal. In satellite relay systems, such as illustrated in FIG. 1, transmit and receive antenna dishes point narrow beams at the geo-stationary satellite. (In “carrier-in-carrier” satellite radio relays, with overlapped up- and down-link frequency bands, as illustrated in FIG. 2, the returned transmit signal to the intended receive signal ratio is nominally the near- to far-end antenna gain ratio multiplied by the ratio of required near- to far-end C/N (carrier-to-noise) ratios.)
VSAT networks typically consist of one or more earth stations with large diameter antennas (called “hubs” or H) that link (to each other, as well as to terrestrial networks) earth stations with N smaller antennas (called “remote stations” or R1-RN). The hub typically modulates a single carrier at a high rate to transmit data, via a signal H, to the remote stations using time division multiple access, while it receives the aggregate signal A containing the relatively low rate data Rk signals from remote stations at different carrier frequencies. Thus, required C/N ratios are typically higher for signals emanating from the hub as compared to those from the remote stations (being nominally in the ratio of their respective data rates).
As shown in FIG. 1, the hub and the remote stations receive the aggregate signal A. The IId signal, within aggregate signal A, is a copy of the Hub's original wideband H uplink signal that has suffered from delays in time, shifts in frequency, changes in amplitude or other distortions (due to satellite transponder's non-linear amplitude and phase responses). At the remote stations the Hd signal is the “desired signal”. At the hub, the Hd signal is an unwanted “echo” signal. The hub must subtract out a replica of the Hd signal from the received aggregate signal (A) to produce the desired composite remote stations' signals plus noise and inter-modulating products.
Since the returned transmit Rk signals, within aggregate signal A, are typically much weaker at the (more numerous) remote stations than the desired Hd receive signal, due to both the lower transmit signal power as well as lower antenna gain, no echo reduction of any unwanted Rk transmit signals is normally required at the remote stations.
While echo cancellation methodologies, as discussed above, have been employed in telephony, such systems cannot be applied wholesale to the satellite communications environment. Each suppression in telephony, such as line cancellation and acoustic echo cancellation is normally limited to 30-35 dB. Such methods are not, however, amenable to satellite echo cancellers because transponder distortion, with both normalized gain and phase approximately quadratic (at sufficient back-off) with respect to amplitude, cannot be approximated as a small-order, e.g., quadratic filter. In addition, due to the large bandwidths and high data rates of modern satellite signals, echo suppression techniques used in telephony are not practical for satellite signals. Thus, there is a need for noise reduction technology in satellite communications that can properly scale, delay and/or distort at least a portion of the transmitted signal to at least partially compensate for echo noise effects.